1. Field of the Invention
The present invention relates to a process of joining galvanized steel sheets, and in particular to a process of joining door frames of an automobile.
2. Description of the Related Art
A galvanized steel sheet to be used in automoibles is treated with an anti-rusting process. A galvanized steel sheet formed by rollers is often used for a automobile door frame. A process of join a plurality of piled sheets by welding is disclosed at WELDING TECHNIQUE, VOL37-2, page 94.about.98, 1989 (ISSN 0387-0197).
In general, in order to prevent blowholes (a defect described later) from forming in the galvanized steel sheet at the point it is joined to another sheet, the following methods are sometimes used;
(1) One method involves stripping away the zinc galvanized on the part to be joined prior to joining.
(2) A second method involves coating onto the part anti-blowhole agent.
(3) A third method involves using a Cu wire (Cu-Si etc.) for a solder to join the parts.
(4) A fourth method involves using a galvanized steel wire of the Fe family as the solder material.
However, the above-mentioned methods present problems as follows;
(1) In stripping the galvanized steel sheet, the number and complexity of the stripping processes increase, and the anti-rust effect decreases. There is both a mechanical method and a chemical agent treatment method for stripping of the galvanized steel sheet. Regarding the mechanical method, it is necessary to shave about 5/100 mm from the sheet, otherwise it is impossible to wholly strip the galvanized zinc. For the chamical agent treatment method, the chemical agent penetrates the clearance between door frames, in the case of overlapped frames, so that the power of the anti-rust treatment decrease.
(2) For the second method i.e. coating with an anti-blowhole agent, it is necessary to coat the anti-blowhole agent many times, and if necessary, to have previously coated the anti-blowhole agent between the plates of the frames. However, it is impossible to remove the anti-blowhole agent from between the plates of the frames after they are joined.
(3) For the third method, using Cu wire as the solder, there is a reduced number of blowholes produced. However, after the joining there is performed a method of electrodeposit coating as treatment of the door frame. The electric resistance of the joint is smaller than that of the galvanized steel sheet because of the Cu solder, so that a thicker layer of the coating adheres to the joint, and the appearance is faulty. Additionally, in general before electrodeposit coating, a phosphatizing of the frame is performed. This phosphatizing treatment is possible on Fe or Zn, but not on Cu.
The electric resistance of the phosphatizing coating is very high, so that the differance of the coating thickness increases more and more.
(4) For the fourth method, using a wire of galvanized steel sheet of Fe family, the blowholes decrease 166 .about.1/10, but are not eliminated.